1. Field of the Invention
The present invention relates to part-time four-wheel drive vehicles the operating mode of which is changeable between a two wheel drive mode and a four wheel drive mode, and more particularly to a part-time four-wheel drive vehicle equipped with a transfer device adapted to change the operating mode of the vehicle drive system from a two-wheel drive mode to a four-wheel drive mode in response to engagement of a hydraulically operated clutch mechanism provided therein and with a split axle drive mechanism in the form of a differential which includes a clutch mechanism disengageable for disconnecting a side shaft of the differential from a non-driven split axle part when the two-wheel drive mode is selected at the transfer device and engageable for connecting the side shaft to the non-driven split axle part when the four-wheel drive mode is selected at the transfer device.
2. Description of the Prior Art
Japanese Utility Model Early Publication No. 59-139424 discloses such a part-time four-wheel drive vehicle as described above wherein the clutch mechanism of the transfer device is arranged to be brought into engagement under control of a solenoid valve in response to operation of a changeover switch while the clutch mechanism of the differential is arranged to be brought into engagement with a predetermined time delay after operation of the changeover switch. In such control of the clutch mechanisms, the delay time for the latter clutch mechanism is determined irrespectively of a time for effecting engagement of the former clutch mechanism. In a cold season, however, the viscosity of fluid under pressure supplied to the former clutch mechanism increases due to fall of the ambient temperature, resulting in an increase of flow resistance of the fluid. This retards engagement of the former clutch mechanism. If the delay time for the latter clutch mechanism is short, the latter clutch mechanism will be brought into engagement prior to engagement of the former clutch mechanism. In such a case, smooth engagement of the latter clutch mechanism may not be effected. In the case that the delay time for the latter clutch mechanism is prolonged to avoid the foregoing problem, prompt engagement of the latter clutch mechanism may not be effected after engagement of the former clutch mechanism in a warm season.